Modern wireless transmitters modulate both the phase and amplitude of a carrier to provide high spectral efficiencies. An amplitude modulated carrier has a non-constant envelope that varies in proportion to the data it encodes. In order for a receiver to properly recover the data from such a carrier, the shape of the carrier's envelope should be preserved as much as possible after power amplification at the transmitter.
To this end, wireless transmitters that perform amplitude modulation generally use a linear power amplifier to increase the power level of the modulated carrier before the carrier is transmitted over a channel to a receiver. In contrast to a non-linear power amplifier, the output of a linear power amplifier is proportional to its input. Thus, the shape of the carrier's envelope is preserved at the output of the linear power amplifier along with the data it encodes. Power amplification is used, for example, to overcome losses of the channel and provide immunity against noise.
Linearity and power efficiency are generally competing performance metrics of conventional power amplifiers. In other words, conventional power amplifiers are generally either highly linear or highly power efficient, but not both. In portable communication devices, such as cell phones and laptops, this tradeoff can result in shorter battery lifetime because the linearity requirement generally takes priority over power efficiency.
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